This invention relates to electro-optical display devices in general and more particularly to a display panel employing electrophoretic dispersions for producing graphic data.
The electrophoretic effect is well known, and the prior art is replete with a number of patents and articles which describe the effect. It is sufficient to state that researchers have been working with the electrophoretic effect for many years. Essentially, the electrophoretic effect operates on the principle that certain particles will become electrically charged and because of being electrically charged, these particles can migrate from a like charged surface to an opposite charged surface. Hence particles which become positively charged will migrate towards a negative surface or terminal or vice versa.
Essentially, many of the prior art references describe various particle materials which exhibit the electrophoretic effect. The effect is associated with a working fluid which includes a dispersion of electrophoretic particles. These particles may be fabricated from an opaque dielectric material such as Titania which particles are suspended in a colored non-conducting suspension medium. The particles are distributed uniformly throughout the suspension medium, and for example Titania particles may be white with the suspension medium being black. When the composite material is subjected to an electric field, the particles are caused to move electrophoretically in the direction of either the cathode or the anode. These particles are deposited upon the respective structure to cause that structure to assume the color of the particle which then would be white. Hence as one can see, by selectively moving the particles, one can produce images based on the migration and orientation of the particles with respect to a charged surface. As indicated, the effect is well known.
For suitable examples of such devices using the electrophoretic effect, reference is made to U.S. Pat. No. 4,093,534 which issued on June 6, 1978 and entitled WORKING FLUIDS FOR ELECTROPHORETIC IMAGE DISPLAY DEVICES, by C. F. Carter et al. This patent provides a working fluid which includes a dispersion of finely divided particles of an opaque dielectric material suspended in a suspension medium. The particles are transportable within the suspension medium under the influence of an electric field. The patent describes various structures which produce a display operating according to the electrophoretic phenonmenon.
Other patents of interest are U.S. Pat. No. 4,298,448 entitled ELECTROPHORETIC DISPLAY issued on Nov. 3, 1981 to K. Muller et al. This patent describes an electrophoretic display which utilizes electrophoretic particles of various pigments which particles are provided with a coating of organic material which is solid at the cell operating temperature but which melts at higher temperatures. The coating material contains a charge control agent to cause the particles to possess a uniform surface potential and hence to allow the particles to move in a more controlled manner.
U.S. Pat. No. 4,285,801 entitled ELECTROPHORETIC DISPLAY COMPOSITION issued on Aug. 25, 1981 to A. Chiang describes a suspension for electrophoretic display systems where the particles are coated with a highly fluorinated polymeric material which acts as a dispersant. Essentially, as indicated in such systems, the particles are chosen to optically contrast with the medium. According, the optical contrast may result from using light reflecting versus light absorbing materials or materials of different colors as above indicated.
Thus in the above example, where white particles are dispersed in a black fluid, the suspension will be grey until application of an electric field. The particles affected by the field will migrate to one surface or electrode giving it a white appearance with the corresponding areas of the opposite electrode or surface appearing black. As one can ascertain from the above noted prior art and various other patents and articles which also have been referenced in the prior art, the electrophoretic display or electrophoretic image displays (EPID) are well known and have been investigated by many researchers. In spite of such investigations there has not been available a display utilizing the electrophoretic effect which has been commercially successful.
An electrophoretic display has many advantages in that the materials are relatively inexpensive, while the image formed remains even when power is removed. When the electrophoretic particles or dye particles are caused to move, as indicated above, to form an image, the image will not erase and remains on the display even upon removing of power. Hence the image must be erased in the same manner as it was created and that is by application to the device of an electric field of opposite polarity.
Thus such displays have a built-in memory in the sense that the images created by the displays do not have to be refreshed such as those images produced by CRT's and other types of display devices. The problems faced by the prior art are many which involve seeking a reliable electrophoretic medium as well as for providing a large size display which is economical to fabricate and which is reliable in operation.
It is, therefore, an object of the present invention to provide am improved electrophoretic display which operates in conjunction with an improved electrophoretic medium. It is a further object to provide an electrophoretic display which is capable of reliable operation associated with a rugged structure which is economical to fabricate.